Reliability of a Compact Carbon Ion Medical Accelerator

Reliability of a Compact Carbon Ion Medical Accelerator

H. Souda1, K. Yusa1, M. Tashiro1, H. Shimada1, A. Matsumura1,

Y. Kubota1, E. Takeshita2, S. Yamada2, T. Kanai1, M. Torikoshi1

1 Gunma University Heavy Ion Medical Center

2 Kanagawa Cancer Center

1

Accelerator Reliability Workshop 2017

19 October 2017

Contents• Introduction of GHMC

• Operation framework

• Stability of the beam parameters

• Operation Statistics

• Severe Troubles

• Future Works

2


19 October 2017

Carbon Ion Medical Accelerator

3

Gunma University (2010~)

Gunma

Carbon 5Proton 12

Oct. 2017

NIRSHyogo

Saga

Yamagata

Kanagawa

• 1974- Clinical Trial of Heavy Ion Therapy by 670MeV/u 20Ne at LBL, Bevalac(Bevatron + Heavy ion linac)

• First heavy ion medical-dedicated accelerator: Heavy Ion Medical Acceleratorin Chiba (HIMAC) in 1994

• Compact carbon accelerator: Gunma University Heavy Ion Medical Center(GHMC)

• >2500 patients were treated from 2010 to 2017• The same model was built in Saga and Kanagawa.• Two facilities are under construction (Osaka, Yamagata)

Osaka

HIMAC (1993~)

7800 m3

320 M$3200 m3

120 M$

National Institute for Radiological Science


19 October 2017

4

66.0m

38.4

m

65m

HIMAC

GHMC

ECR ion source

Injector Linac

Synchrotron

Compact Carbon Ion Accelerator120m

Design Concept:• Compact, Low Cost→ 1/3 size, 1/3 cost• Comparable performance

to HIMAC→ 5Gy(RBE)/min, 25cm range→1x109particle/s, 400MeV/u• Stable and easy for

operation→ Automated start/shutdown


19 October 2017

Ion Source

Gas Inlet

Microwave

feed port

Ring Permanent

Magnet

Iron Yoke

Ground

Electrode

Sextupole

Ceramic Insulator

NIRS-ECR(Electric Magnet)36m2 (incl. RF/power source)

KeiGM (Permanent Magnet)2m2 (incl. RF source)

5

M. Muramatsu et al., Rev. Sci. Instrum. 76, 113304 (2005)

Outside Dimension 295 x f310 mmPlasma Chamber 270 x f50 mmMagnetic Field Max. 0.8 T (ECR 0.37T)

Ext. Electrode Molybdenum f 6 mm

RF Amplifier Travelling Wave Tube (TWT)NEC LD79X75A1 (Typ. 200W)

RF Frequency 10GHzGas species Methane (CH4) 0.02 sccmIon Species C4+ Typ. 200mAExt. Voltage 30kV (10keV/u)

Maintenance:Changing Ext. Electrode (every 6 mo.)Overhaul of TWT Amp. (every 2 yr.)


19 October 2017

Injector Linac

Y. Iwata et. al., Nucl. Instrum. Meth. A569, 685 (2006)

HIMAC Alvarez DTL (100MHz)Diameter 2mLength 24m

APF IH-DTL (200MHz)Diameter 35cmLength 3.5m

6

Elec. Magnet Focusing Alternative Phase Focusing

RFQ IH-DTL

Particle C4+ C4+

RF 2kW TRA

20kW Tetrode

200kW tetrode

5kW TRA

50kW Tetrode

500kW tetrode

Operation Power 140 kW 360kW

RF Frequency 200MHz 200MHz

Ext. Energy 0.6 MeV/u 4 MeV/u

Tank Length 2.5 m 3.5 m

Max. Field 23.6 MV/m

(1.6 Kilp.)

23.6 MV/m

(1.6 Kilp.)

Maintenance:Changing Tetrode (every year)Changing Carbon foil Stripper (~ 2yr.)

World’s first practical APF-IH Linac


19 October 2017

Synchrotron

0 5 10 15 200

5

10

15

Be

ta &

Dis

pe

rsio

n f

un

ctio

n [

m]

s [m]

HIMAC2 Rings(USY+LSY)C=129.6m 800MeV/u

GHMC1 RingC=63.3m 400MeV/u

7

T. Furukawa et al., Nucl. Instrum. Meth. A562, 1050 (2006)

Ions C6+

Injection 4 MeV/uMax. Energy 400 MeV/uBeam Intensity Typ. 1.3×109 ppsRepetition 0.3 Hz (typical)Circumference 63.3 mMax. Field 1.5 TInjection Multiturn(～ 25 turn)

Extraction Slow Ext. (~1s)

Maintenance:Power Source (every year)Cavity RF Amplifier (every year)Cleaning of strainer of the magnet(every year)


19 October 2017

Irradiation System

8

# of course 4 (AHC, BHC, BVC, CVC)

Irradiation Method Broad Beam (Wobbling)

Field Size 15 cm x 15 cm

Spread Out Bragg Peak 2 - 14 cm

Range 25cm (400MeV/u)

Dose Rate 2 Gy/min

AHC

BHC

BVCCVC


19 October 2017

Numbers of treatment


19 October 2017 9

20~60 Patients / month

20~60 irradiations / day(Avg. 12~14 Irradiations per patient)

20 or 30 min / irradiation(incl. patient positioning)

8 ~ 9 hour /day= ~20 irradiations * 3 rooms= 60 irradiation /day

Operation Time

10

0 7:00 7:30 8:40 12 13 15 16 17 23 24

Monday Weekly Inspection Machine Study Shutdown

Tuesday Startup Machine QA Treatment Rest Treatment Patient QA Shutdown

Wednesday Startup Machine QA Treatment Rest Treatment Experiment Shutdown

Thursday Startup Machine QA Treatment Rest Treatment Experiment Shutdown

Friday Startup Machine QA Treatment Rest Treatment Patient QA Shutdown

Saturday Startup Experiment or Maintenance shutdown

Sunday Startup Experiment or Maintenance shutdown

Operator Shift A (3)

Operator Shift B (3)


19 October 2017

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov DecAnnual maintenance for 1 month

hour 10

minute 0 5 10 15 20 25 30 35 40 45 50 55 0

Room A

Room B

Room C

9

4 treatment days / week

20 or 30 min /patientAccelerator is standby during patient positioning

QA Schedule

11

Frequency Items

Daily(Startup)

Patrol (Visual inspection)Confirmation of Beam axis in the treatment room

Beam Current after Ion Source, Linac, SynchrotronDose/Beam current calibrationSimplified beam range measurement w/ block phantom

Weekly COD in synchrotronAlignment check in the treatment roomFull stroke drive of treatment room devices

Monthly Respiratory Gating SystemDetailed beam range measurement w/ water phantom

Annually Power source Inspections by the manufacturerBeam line alignment checkInterlock SystemChange of extraction electrode in the ion source (half yr)Change of Tetrodes for Linac RF Amplifier


19 October 2017

0

50

100

150

200

250

300

350

400

450

42015

5 6 7 8 9 10 11 12 12016

2 3 4 5 6 7 8 9 10 11 12 12017

2 3

Tim

e[h

]

Treatment Measurement Experiment Downtime

Operation Time

12

Annual Maintenance

Annual Maintenance


19 October 2017

~4200 hours operated

(1400 hours for treatment)

Relation with machine venders


19 October 2017 13

Vendor(Mitsubishi Electric Co.)No engineer on siteMaintenance team/design team

Hospital (Gunma University)7 Medical PhysicistsIncl. 1 Accelerator Physicist

Operation contract:# of operators and operation timeNo threshold of availability

Maintenance contract: On-call maintenance for failure

Operators (Mitsubishi Electric)10 operators

Normal Operation

Daily QAPatient-related measurement

Startup/ShutdownDaily QA

In Case of trouble…

InvestigationRecovery Check

Initial investigationFirst aid (reset,restart)Change to Spare

Detailed investigationRepair PlanningRepair Work (with parts vender, SHI, IDX, Kudo, Thamway)

Annual Maintenance

Decision of Repair/Ignore for small troubles→ Tradeoff of cost and reliability.

Stability of beam current• Pulse to pulse: 10% by fluctuation of ion source

• Ion Source is enough stable in a long term.

14


19 October 2017

Jumps are caused by drift of offset of a mass flow controller of CH4 gas

Stability of beam current

15

Legal permission: 1.58x109pps

CFS Change CFS Change

IHL Voltage Adjust

IHL Voltage Adjust

• Synchrotron beam current depends on the injection energy…affected by carbon foil stripper

Acceptable

Best


19 October 2017

CFS Change

Availability

• Availavility = 1 −𝑀𝑇𝑇𝑅

𝑀𝑇𝐵𝐹+𝑀𝑇𝑇𝑅= 1 −

𝑇fail

𝑇normal+𝑇fail• MTTR: Mean Time to Repair = Time with failure

• MTBF: Mean Time Between Failure = Time without failure

• Failures(>1min) are recorded manually by operators

16


19 October 2017

92.5

97.4

98.4 98.598.1

99.1 99.4

97.7 97.798.2 98.2

90

91

92

93

94

95

96

97

98

99

100

2010 2011 2012 2013 2014 2015 2016

Perc

enta

ge

Treatment Availability

Overall Availability

HEBT PSSnow

Linac AmpSolenoid PS

Water LeakX-ray

Severity of troubles

17

0

200

400

600

800

1000

1200

1400

0 min ～5min ～10min～30min～60min ～3h ～6h 6h~

Nu

mb

er o

f fa

ilure

Failure Time

Inj63%

Syn6%

HEBT6%

Irrad16%

Pos9%

Level 0 (No Delay)

Inj19%

Syn7%

HEBT8%Irrad

44%

Pos22%

Level 1 (Delay <5min)

Inj10%

Syn8%

HEBT6%

Irrad63%

Xray13%

Level 2 (Delay <3h)

Inj33%

Syn0%

HEBT5%

Irrad29%

Pos33%

Level 3 (Delay >3h)

Single Discharge

Continuous Discharge

CalibrationFailure

Control System

MechanicalDevices

Tetrode

X-rayDetectorFailure


19 October 2017

Severe Troubles

18

Time to repair > 3h is regarded as a severe trouble.

DateTime to Repair

Treatment Delay

Section Description

2010/7/17 Fri 1d 1d Injector Burnout of RFQ-HPA feedthrough capacitor

2010/8/12 Thu 9h 1d Injector Breakdown of RF Amp of ion source

2011/10/13 Thu 10h 1d Injector Discharge of Einzel lens

2012/4/30 Mon 3d 6h Injector Frequent trip of RFQ-HPA

2012/5/15 Tue 8h 1d Synchrotron PLC error of synchrotron bending magnet

2012/12/7 Wed 3d 2d Injector Burnout of IHL-HPA Amplifier

2013/7/30 Tue 10h 7h HEBT Voltage drop of HEBT-QM control circuit

2014/3/30 Sun 14h 0 Injector Discharge at IHL-HPA input loop

2014/5/29 Thu 13h0(Change

rooms)X-ray Filament break of X-ray Tube

2014/10/30 Thu 17h 4h Injector Diode failure (open) of LEBT Solenoid PS

2014/11/4 Tue 13h1h(Change

rooms)Control Breakdown of optical diode of Room B accelerator interface

2015/3/16 Mon 9h 0 X-ray Discharge of FPD

2015/4/20 Mon 9h 0 X-ray Discharge of FPD

2016/5/30 Mon 2d 0 HEBT Power source failure caused by water leakage

2016/11/28 Mon 5h 0 Injector Breakdown of penning vacuum gauge

2017/4/26 Wed 12h 1h Control Infection of computer worm to control system

After 2013, treatments did not stop for 1 day or longer.


19 October 2017

Trouble 1:Einzel lens discharge

19

Date 13 Oct 2011 20:00

Event Einzel Lens (18kV) continuously discharged

Cause Insulation degraded by high humidity (~70%)

Delay 1 day

Repair • Replace feedthrough• Decrease humidity by air

conditioning• Improve ventilation around

feedthrough

Initial failure (combination with environment)

Trouble 2:Penning Gauge Failure

20

Date 28 Nov 2017 10:35

Event Penning vacuum gauge wasbroken and wrongly gave a vacuum pressure failure.

Cause Carbon was deposit inside the gauge

Delay 0 hour(5 hour to repair)

Repair Change the gauge and conditioning the ion source

Pre-indication

Small change of the readout of vacuum pressure was observed from 1 month before


19 October 2017

Aging deterioration(with inappropriate handling)

Trouble 3: Computer Worm Infection

21

Date 26 Apr 2017 8:00

Event Accelerator control client froze with OS error message. They were infected by Conficker.B worm.

Cause Infected from positioning system by USB stickOS (win XP) was not upgraded

Delay 1 hour (12h to repair)

Repair Fixed by antivirus program→Changed to spare or re-installed

Pre-indication

svchost error message was observed


19 October 2017

Insufficient preventive maintenance (update)

To prevent severe failure• Decrease ‘sudden’ trouble

– Trouble prediction from similar troubles

– Share trouble report with other facilities

• Find indications of trouble

– Investigate root cause and repair

– Machine Time for trouble shooting

• If trouble happened, prevent recurrence

– Set priority to repair for severe trouble

– Evaluation of criticality – FMEA approach

22


19 October 2017

Multi-Facility collaboration• Share the trouble reports with other facility (Many common parts

among carbon facilities)

• Workshop – Particle Therapy Facility Operation and Maintenance

• 2014 – 2017 NIRS, Gunma University (~100 participants)


19 October 2017 23

Participant of 4th W-PTFOM

Machine Study Time• Machine Study time (18:00-

24:00 of Monday) is used for Trouble shooting as well as commissioning.


19 October 2017 24

Trouble Shooting

33%

Parameter Optimization

27%

Experiment Preparation

22%

Commissioning18%

• Parameter Optimization Injector, Synchrotron, HEBT

290MeV/uspill

• Trouble Shooting

Investigate indications of trouble to find the root cause

Chattering of Extraction Gate Signal→ loose connection was corrected→ FPGA logic improved

small spike at start of flattop

Optimization ofCOD, RF pattern

180h/year

User’s side FMEA approach• We are planning to use Failure Mode and Effect Analysis(FMEA) to

decide priority of the ‘happened’ troubles using actual delay time and failure frequency.

25


19 October 2017

Effect

16: Delay > 1h

8: Delay <1h

4: Delay <30min

2: Delay <10min

1: Delay <1min

Frequency

16: 2~ in 1day

8: Once in 1day

4: Once in 1week

2: Once in 1mo

1: Once in 1yr

EffectScore

16>1h

16 32 64 128 256

8<1h

8 16 32 64 128

4<20m

4 8 16 32 64

2<10m

2 4 8 16 32

1<1m

1 2 4 8 16

11/yr

21/mo

41/w

81/day

162~/day

Freq.Score

Criticality Score Matrix

Need Immediate Repair

Need Repair in next maintenance

Carefully Watch and Investigate

No action

Example of User’s side FMEA


19 October 2017 26

EffectScore

16>1h

16 32 64 128 256

8<1h

8 16 32 64 128

4<20m

4 8 16 32 64

2<10m

2 4 8 16 32

1<1m

1 2 4 8 16

11/yr

21/mo

41/w

81/day

162~/day

Freq.Score

Penning Gauge FailureEffect: <1h = score 8

Freq: 1/month = score 2Total score: 16 = highly intolerable

→ Need repair

Single Discharge of linacEffect: <1m = score 1

Freq: 1/week = score 4Total score: 4 = mostly tolerable

→ OK

Summary• GHMC has operated a compact carbon accelerator for 7 years and has

treated more than 2500 patients. Nowadays, the treatment availability was more than 99%.

• Severe troubles which caused a downtime of max. 2 day occurred in early years.

• Based on the experience of severe troubles caused by incorrect actions to the trouble indication, FMEA approach will help us to decide the action.

27


19 October 2017

Acknowledgement

28


19 October 2017

The authors would thank to:

Operators of Mitsubishi Electric CorporationN. Ikeda, K. Takeuchi, N. Matsumura, S. Mochizuki, H. Haba, T. Takeuchi, K. Funatsu, M. Ishii, A. Omae, Y. Higuchi

Accelerator Engineering CorporationI. Kobayashi, Y. Kano

and all participant of ARW2017,

Merci!

Supplementary slides

29


19 October 2017

Carbon Ion Cancer Therapy

30

• 1946 R. R. Wilson proposed medical use of proton (advantage of dose distribution by Bragg peak)

• Heavy ion beam: advantage on dose distribution and relative biological effectiveness(RBE)

• Needs 300-800 MeV/u beam for 25-30cm range in human body

• 1974- Clinical Trial of Heavy Ion Therapy by 670MeV/u 20Ne at LBL, Bevalac (Bevatron + Heavy ion linac)

• 1994- Clinical trial of carbon ion therapy by HIMAC

• 2010- Carbon Ion therapy by commercial accelerator (GHMC, etc.)

Required range (25~30cm)

Max. energy of 300-800MeV/u is needed

Sumitomo Heavy Industry

Injector Commissioning Team

Cooperation for Commissioning• GHMC Compact Accelerator is the first Japanese commercial heavy ion medical

accelerator based on the NIRS model.

• Accelerator specialists in GHMC and the accelerator designers in NIRS has collaborated for the commissioning.

31

Mitsubishi Electric Corporation

On-site Commissioning Team

Accelerator Design Team

National Institute for Radiological Science(NIRS)

Gunma Support Team

Dr. Koji NodaDr. M. Muramatsu

Dr. Y. IwataDr. T. Furukawa

Gunma UniversityHeavy Ion Medical

CenterPhysics Group

Prof. S. YamadaDr. K. Torikai

Prof. T. KanaiDr. K. Yusa

Dr. M. Tashiro


19 October 2017

Milestones of Commissioning

32

Milestone Date Effort

Factory Test Cavity, Magnet, Power Source

Site Installation 19 Aug 2008 Assembling, Alignment

On-site Test 12 Aug 2009 Individual operation

Linac First Beam 19 Aug 2009 Aging, Re-alignment

Synchrotron 400MeV/u Acceleration 25 Aug 2009 Optimization of injection param.

First Beam on Treatment Room 5 Sep 2009 Extraction and HEBT optimization

Radiation Facility Legal Inspection 21 Oct 2009 Stable operation

Clinical Commissioning 19 Dec 2009 ~ Reproducible beam parameter

1st treatment 16 Mar 2010 Check of interlock


19 October 2017

Linac First Beam

33

Acceleration in the Injector Linac is very sensitive to its accelerating voltage. At first, the beam signal was not observed because the pickup voltage has a systematic error of 20%. The first beam was observed by temporary installed fluorescent screen monitor.

0

0.05

0.1

0.15

0.2

0.25

0% 20% 40% 60% 80% 100%

Bea

m C

urr

ent

[uA

]

RFQ Voltage [Rel. to Nominal Value]


19 October 2017

Synchrotron Acceleration

34

The first turn beam was observed by a movable fluorescent screen.The circulation and the acceleration was observed by a DC-CT (beam current monitor).

Bending Magnet Current

Beam Current

injection

acceleration


19 October 2017

400MeV/u Extraction

35

• Slow extraction was realized by RF acceleration in GHMC system. The first extracted beam was observed by a beam loss monitor (scintillation radiation detector)

• After the transport to the treatment room, beam test was carried out in the legal inspection of the radiation shielding.


19 October 2017

Beam Quality Improvement• An actual accelerator has small errors compared with designed value (Alignment, Magnet

length, gap).

• Betatron tunes were precisely optimized for the stable extraction. Closed orbit distortion was corrected for a minimum beam loss.

• There is a strict time limit because the irradiation system requires to fix the accelerator beam parameters as soon as possible!

36

Beam spike Beam Loss

Beam spike

Set betatron tunefar from resonance

Smoothing of COD corrector(Steering Magnet)

R (Avg. COD)Correction


19 October 2017

Trouble Shooting• A number of troubles occurs in the commissioning period. Trouble

shooting is very important for the availability and safety in the treatment operation.

• Sample of hardware trouble: frequent discharge of ion source. Shape of the anode and cathode electrode was changed in the commissioning.

• Sample of software trouble: lack of RF and magnet pattern resolution. This result in the beam spike extraction. The program of pattern compilation was updated.

37


19 October 2017

Trouble 1: RF Amp burn out

38

Date 7 Dec 2012 20:00

Event IH-DTL Amplifier filamentcables and mounter were burned.

Cause Total-reflection on the tetrode → short of the input circuit

Delay 2 days

Repair Change tetrode, mounter electrode, and cable. Insulation improvement

Pre-indication

Temporary increase of reflected RF power (2h ago)

Trouble 2:HEBT-QM Failure

39

Improvement: connector was soldered directly

Date 30 Jul 2013 20:58

Event H-CL-Q2P down by drop of control voltage (5V→4.2V)

Cause Voltage drop in the cable and loose connection

Delay 7 hours

Repair Loose connection was recovered by itself.wiring was totally improved in 2015.

Pre-indication

Temporary restart of a microcomputer (resettable)

Trouble 3: Beam Axis Jump

40

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

6:00 9:00 12:00 15:00 18:00

380BHC11/17x

11/18x

11/19x

11/20x

Date 20 Oct 2015 11:00

Event Beam axis jumped to >3mm (not acceptable). Need HEBT orbit correction.Sometimes does not occurOnly in 380, 400MeV/u

Cause D/A converter of Bump PS did not work on 5th bit (3%)That made COD error.

Delay 1 hour (3 month to find!)

Repair Changing D/A board

Pre-indication

Change less than <2mm had occurred since 14 Oct.

Preparation for disaster

41

Grade 1 (tiny) Grade 2-3 (weak) Grade 4- (strong)

Continue Operation. continue operation after safety patrol (~10min)

Make patrol and submit legal report for radiation safety.

5~10 times in a year 1~3 times in a year Once in 6 years

GHMC experienced Only one strong earthquake (2011.3.11 East Japan Earthequake)No damage was observed and change of the COD and beam axis was smaller than 1mm.

Momentary (<0.1s) Short Time (~15min) Long Time (~1day)

Continue Operationif there are no warning

Restart computers, control system. Vacuums are maintained by UPS

1-2 times in a year Once in 2-3 years Usually does not occur

Earthquake (There are many earthquakes in Japan)

Blackout (Gunma has many thunders in the summer)

Hospital Power Generator will supply minimal system (vacuum, computer)

Blackout• Gunma is an area with frequent thunder (once in 2-3 days in the

summer). Momentary voltage drops occur several times in a year.

• The vacuum system stopped by the voltage drop and need 1-2 hours to be recovered. Since 2012, the vacuum system has been fed by UPS.

• University hospital have a on-site power generator to avoid a blackout for a long time. But the threshold of the relay was not set properly and caused a blackout of 12 min in the midnight of 2 August 2016. The treatment was delayed for 1 hour to restart the computers etc.

42


19 October 2017

事例1: 絶縁碍子の劣化• 2011.10.13 イオン源アインツェルレンズで放電が頻発しビーム出力不能。1日治療停止。数日前から何度か放電が起きていた。

• 現場調査した所、電圧導入端子の絶縁碍子が湿気で緑青を吹いていた。

• 碍子を清掃して応急措置→メンテナンスで交換。

• 加速器室の湿度を除湿で下げる・カバーの通気性を改善するなどの改造を実施

43


19 October 2017

事例2:アンプ焼損• 2012.12.7 20:00 IHL-HPAが非常停止。現場点検したところ、トランス付近のケーブル、マウンタ電極が焦げていた。2日治療停止。

• 真空管(RS2042SK S/N643948)自体の特性変化によりRF全反射し、中性点付近に大電力が集中し短絡したと思われる。

• 真空管交換・電極交換・絶縁強化で復旧。

44


19 October 2017

事例3: 真空管耐圧劣化• 2013.6.1-立ち上げ中の真空管放電(トリップ)が毎日1回程度起こる

• 2013.6.26(水) 立ち上げ時に5回トリップし治療開始20分遅延。電流波

形を確認した所、スクリーングリッドの電流増加を確認。耐圧劣化と判断し交換を決定

• 2013.6.29真空管交換で復旧。

45


19 October 2017

事例4:入射器アンプ放電• 2014.3.30 13:20 週末実験中IHL電圧安定度異常、ビーム出力不可。低出力では正常だが、定格電圧の30%程度まで上げると異常となる。

• 現場調査した所、異常時にはアンプ下部から放電音がしていた。メーカー(住友重工)の指示を受けつつ分解し、放電部を清掃・絶縁強化(カプトンテープ重ね貼り)→改善したが50%程度で放電→放電部研磨・絶縁再強化で正常化。

• 週例点検日に対処完了したため治療遅延なし

46


19 October 2017

事例5: ソレノイド電源故障• 2014.10.31 10:34 治療準備中、LEBTソレノイド電源で重故障(過負荷異常)発生。異常リセット後起動可能となったが、12:27に定格運転(281A)不能となった。

• 13:30より、180Aなら安定運転できることを確認し、集束系調節によりビーム量を確保する調整を開始。通常の1/4まで回復。標準測定が1%低い。臨床上許容内と確認の上、当日の呼吸同期照射は中止、それ以外を実施。

• 23:50 メーカー来所し調査。電源のダイオードがOPENモードで故障したことで残り系統のIGBTに過負荷がかかっていた。部品交換で復旧。

47


19 October 2017

事例6: 電源制御電圧低下• 2013.7.30 20:58 共通ラインのH-CL-

Q2Pの制御電源電圧(5V)低下により電源運転不可となり、ビーム出力不能

• 翌日13時よりメーカー(仙台の工藤電機)来学により調査開始。調査中に接触不良が解消し、治療再開。治療7時間遅延。

• 以前から一時的な電源異常は発生していたが、ケーブル抜き差しなどで回復したので本格処置していなかった。

• ケーブルの接触不良防止の為固縛処置したが再発し、2015年6月に全数ケーブル交換して恒久処置完了

48


19 October 2017

事例7: 謎のビーム軸ズレ• 2015.10.14 治療中にビーム軸が3mm超過(即時治療停止レベル)。朝の軸確認では1mm(正常範囲)だった。異常表示はなし。シンクロトロン再起動で2.7mmになり治療実施。

• 前日に冷却水流量異常が出ていたため流量調整・ストレーナ清掃するも効果なし

• 2015.10.20 ビーム軸3mm超過。昼休みにHEBT軸調整して治療は実施。

• 380MeV、400MeVで発生、290MeVでは影響なし→冷却水温度の影響か？→調整したが変わらず

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謎のビーム軸ズレ2• ビームの変化：CODが不均等に変化している→ステアリング磁石の問題か？→個別に変動させてみたが再現せず

• 2016.1.10 定期点検中、S-BME3(出射バンプ)のD/A変換での出力不良(ビット抜け)が見つかった。該当ビットを使用する380MeV,400MeV

のOPFの場合のみ電圧が3%下がっていた。部品交換で復旧。

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0 1 2−4

−2

0

2

4

mm

PM1X

PM2X

PM3X

PM4X

PM5X

PM6X

0 1 2−4

−2

0

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mm

PM1X

PM2X

PM3X

PM4X

PM5X

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19 October 2017

事例8: 謎のビーム出力不能• 2014.8.14 出射セプタム電極電源(S-

ESD)外部インターロック2でビーム停止。真空インターロックだが、真空度は正常。インターロック端子をバイパス(短絡)させて運転するもビーム周回せず。

• 加速器室現場を見ると、真空ゲートバルブが閉まっていた(これに当たってビームロスしていた)。原因はPLCのメモリ内容エラー(放射線影響)。PLCリセットで復旧。遅延2時間。

• 異常検出できるよう2015.4月に改修したが、それまでにも3回発生した(2-30分遅延)。改修後は5分で復旧できるようになった。

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事例9: 水漏れによる電源故障• 2016.5.30 7:15 始業点検中、空調機械室で水漏れを発見。下階のH-CL-ST1X電源盤が水濡れ、非常停止となっていた

• 乾燥措置し、16:00に運転開始したが、23:30に遠隔制御不能となった。

• 再発に備えて、加速器マシンタイムで、当該電源は出力0のため電源出力なしでもビーム軸が正常であることと、もし再発した場合に制御からの除外で運転可能なことを確認してあった。

• 翌日の治療は現場運用・制御除外で実施し遅延なし。

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水濡れ故障2• 翌日治療終了後にメーカー調査。電源への指令が正常に届いているか(電源入力側)を調査したが正常。電源出力側に異常があり制御がNot Readyと認識して指令を出していなかったことがわかり、端子台を調べた所水濡れによる短絡が見つかった。交換で復旧。

• 2012年にも水漏れで濡れていたので、その影響かも

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事例10: 治療台誤動作• 2013.6.13 CTシミュレーション室1でのリハーサル中、治療台が患者を乗せたまま原点復帰動作(Roll 25度)した。固定具を着けていたので落下しなかったが、重大事故の可能性もあった。

• それ以前から動作停止などの不調が見られていた。

• 治療台ペンダントの通信がシリアル通信(1つの信号線のHi/Loの時間変化で信号を伝達)しており、この線が断線していた。誤動作防止用のバースイッチの信号もシリアル信号線に乗っていたため、安全措置が機能していなかった。

• インターロック信号は別個にハードワイヤーで管理すべき

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Reliability of a Compact Carbon Ion Medical Accelerator

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